1. Field of the Invention
The present invention relates to a method for measuring an etch-ending point, and more particularly, to a method for measuring an etch-ending point during a wet etch process.
2. Discussion of the Related Art
An etching process is usually carried out for forming a variety of patterns during a semiconductor device manufacturing process. It can be used to etch a semiconductor layer, an insulating layer, or a conductive layer. Conventionally, etching processes are classified into processes using either a dry etch method or a wet etch method.
For both the dry etch and the wet etch methods, one important factor is the ability to decide when a layer subjected to etching has been etched to the desired thickness. In order to assure the success of the semiconductor device manufacturing process which requires minute patterns, it is essential to be able to decide accurately when each of the layers subjected to etching has been etched to the desired thickness during the etch process for forming the respective patterns. In other words, it is essential to accurately measure an etch-ending point during an etch process.
The conventional methods for measuring an etch-ending point will now be described for both the dry etch process and the wet etch process.
For the dry etch process, the widely adopted etch-ending point measuring method is a light wavelength method. When the dry etch of a silicon oxide layer (SiO.sub.2) is performed by using an etchant CF.sub.4, the etchant CF.sub.4 reacts to the silicon oxide layer to produce a by-product such as CO.sub.y F.sub.x. A light beam, such as laser, is then projected upon the silicon oxide layer which is reacting with the etchant CF.sub.4. When the light is reflected from the silicon oxide layer and the by-product resulting from the reaction, the intensity of the reflected light at specific wavelengths will vary in accordance with the degree of etching completed of the silicon oxide layer. The intensity of the reflected light at a specific wavelength is measured during the etch process and compared to a certain pre-determined intensity value set by prior experiments. Thus, when the measured intensity of the light is equal to the pre-determined intensity value, the etch-ending point has been reached.
The above described light wavelength method for measuring the etch-ending point is also used in conventional chemical-mechanical polishing (CMA) process. The CMA process etches a layer to a desired thickness by milling the layer subjected to the etch process.
For the wet etch process, three conventional methods for measuring the etch-ending point are often used. For the purpose of discussion, it is assumed that an insulating layer is wet-etched to form a desired pattern in a structure having the insulating layer on a semiconductor layer.
The first method is a color method. After putting this structure into a bath filled with an etchant and conducting the wet etch for a period of time, the resultant structure is taken out of the bath to determine the etch-ending point by examining the surface color of the resultant structure. If the insulating layer is etched to the desired thickness, the surface color of the structure would differ from that before conducting the etch, because the color of the underlying semiconductor layer is different from that of the insulating layer.
This color method is mainly used in a partial etch process. For example, it is used when the insulating layer is etched a thickness of 4000.ANG.-5000.ANG. only.
The second method is a surface tension method. A structure comprised of a semiconductor layer, such as silicon, and an insulating layer is put in a bath filled with a wet-etchant and subjected to the wet etch. Then, the resultant structure is taken out of the bath and the surface is sprayed with water. If the insulating layer is thoroughly etched, the surface of the silicon would be exposed. Accordingly, the silicon surface may be slightly stained with the water due to the surface tension of the silicon. If the insulating layer is not thoroughly etched, the sprayed water does not adhere to the surface, instead it flows over the surface. Thus, the etch-ending point can be measured.
The third method for determining the etch-ending point uses a thickness meter. Initially, a structure comprised of a semiconductor layer and an insulating layer is put into the bath and subjected to the wet etch for a period of time. Then, the resultant structure is taken out of the bath, and the thickness of the structure is measured by using a thickness meter. If the insulating layer is thoroughly etched, the measured thickness of the structure equals the thickness of the semiconductor layer. If the insulating layer is not thoroughly etched, the measured thickness of the structure would be larger than the thickness of the semiconductor layer.
All three etch-ending point measuring methods for the wet etch process are visual methods, which can be difficult to apply when the structure has patterns. Since the etch process employed for manufacturing semiconductor devices is frequently used to form patterns, and the patterns are often of a minute nature, it is almost impossible to visually measure the etch-ending point under those circumstances.
Conventionally, in order to actually apply the above-described methods during the manufacturing process of the semiconductor devices, a test pattern with an enlarged configuration is utilized. In other words, the same manufacturing process is simultaneously performed upon the pattern when the process is used to manufacture the semiconductor devices. The test pattern is a considerable enlargement of the actual structure, so that the above three etch-ending point measuring methods can be applied. The etch-ending point of the actual structure can be only estimated by using the color, surface tension, and thickness of the test pattern.
This technique of using the etch-ending point on the test pattern to provide an estimation for the etch-ending point of the actual structure has several problems. First, it has an inherent risk of inaccuracy of the etch-ending point for the actual structure, and such inaccuracy usually causes failures of the manufacturing process.
In addition, due to the large size of the test pattern involved and the large amount of the chemical material (such as the etchants) required, the whole manufacturing process becomes very complicated and expensive.
Furthermore, the structure is taken out of the bath one by one to either determine the color and surface tension, or measure the thickness with the meter, thereby increasing the processing time.
Therefore, the conventional etch-ending point measuring methods for the wet etch process are not as effective in various aspects as the measuring method for the dry etch process. However, during the manufacturing process of the semiconductor devices, it is well known that the wet etch is preferred to the dry etch under certain circumstances. Consequently, an effective etch-ending point measuring method for the wet etch process is needed.